1. Technical Field
This invention relates to a three-phase induction motor control method for performing motor drive control and regenerative control based on digital values.
2. Description of the Related Art
Recent control of three-phase induction motors is in many cases performed by a vector control method of controlling the instantaneous value of motor stator current and generating a torque equivalent to that of a shunt DC machine.
Using vector control of this type to perform motor drive control and regenerative control of a three-phase induction motor is disclosed in e.g. Japanese patent application Laid-Open No. 59-14385.
If an induction motor is subjected to vector control in cases where velocity control is performed up to a region of high rotational velocity, as in a spindle motor of a machine tool or the like, or in cases where control is performed to weaken excitation in accordance with a torque command at a constant rpm, the torque command, excitation current, secondary current and slip frequency become non-linear due to such effects as the secondary leakage impedance and core loss of the motor. As a result, the output torque also becomes non-linear with respect to the torque command, so that a linear relationship between the torque command and output torque cannot be accurately maintained.
Furthermore, since the output of an induction motor generally varies in proportion to the square of the voltage impressed upon the motor, the output will fluctuate when the AC input voltage undergoes a large variation. In the vector control method, a measure devised for preventing such a fluctuation in output entails varying the amount of maximum slip in dependence upon the AC input voltage. Nevertheless, satisfactory results are not obtained in terms of holding the output of the induction motor constant.
At time of regenerative braking, moreover, the power-factor fluctuates due to the capacity of a phase advancing capacitor on the input side, and there is an increase in power supply distortion and reactive power.